The present invention relates to an improvement in an ejector for generating a negative pressure and also pertains to an improvement in a negative pressure supply apparatus using an ejector.
In general, an automotive brake system is provided with a pneumatic booster to increase braking force. The pneumatic booster generally uses the engine intake system as a negative pressure source. That is, the engine intake (negative) pressure is introduced into a negative pressure chamber to produce a differential pressure between the intake pressure and the atmospheric pressure, thereby generating thrust in a power piston to assist the brake system with operating physical force.
This type of pneumatic booster suffers from the problem that because it utilizes the engine intake (negative) pressure, the pneumatic booster may be incapable of obtaining a sufficiently high negative pressure (degree of vacuum) under engine running conditions where the engine intake vacuum pressure is low, e.g. immediately after the engine has started cold. In such a case, the servo power may be reduced. The reduction in the servo power becomes a problem in the case of small-sized engines with a small piston displacement (intake air quantity). Under these circumstances, there has heretofore been proposed pneumatic boosters using an ejector to increase the negative pressure to be introduced into the negative pressure chamber [see Japanese Patent Application Unexamined Publication (KOKAI) Nos. Sho 59-50894 and 60-29366].
The ejector has a nozzle and a diffuser disposed downstream of the nozzle. A negative pressure outlet is provided between the nozzle and the diffuser. When a gas is allowed to flow from the nozzle toward the diffuser, a high-speed jet is produced, whereby a high negative pressure can be generated at the negative pressure outlet.
There has recently been an increasing demand for lean-burn and cylinder injection engines to reduce exhaust emissions and increase fuel economy. In these engines, however, the degree of throttling achieved by the throttle valve is low because of the structure thereof, and hence it is difficult to obtain a high intake negative pressure. Therefore, there is an increasing demand for an ejector capable of generating a high negative pressure with a relatively low intake negative pressure.
Regarding a negative pressure supply apparatus for supplying a negative pressure to an automotive brake system, it is required to generate a high negative pressure with a low intake negative pressure and to recover the negative pressure in the negative pressure chamber of the pneumatic booster rapidly after the negative pressure in the negative pressure chamber has been consumed by the operation of the brake system. Accordingly, the ejector is required to be capable of obtaining a high negative pressure (degree of vacuum) with a low intake negative pressure and, at the same time, capable of obtaining a sufficiently large suction air quantity.
The present invention was made in view of the above-described circumstances.
An object of the present invention is to provide an ejector capable of obtaining a high negative pressure with a low intake negative pressure and, at the same time, capable of obtaining a sufficiently large suction air quantity.
Another object of the present invention is to provide a negative pressure supply apparatus capable of supplying a stable negative pressure by using the ejector.
The present invention is applied to an ejector wherein a diffuser is disposed downstream of a nozzle, and a suction port is disposed between the nozzle and the diffuser. According to the present invention, the nozzle and the diffuser are combined together to form a substantially single Laval nozzle. Moreover, the inlet of the diffuser is enlarged in width so that the side walls thereof extend approximately parallel to each other from the opening of the suction port.
With the above-described structure, the Laval nozzle allows the flow velocity at the throat portion to reach the sound velocity even when the intake negative pressure is low. Thus, a high negative pressure can be obtained. Further, because the inlet of the diffuser is enlarged and extended parallel to the axis of the diffuser, the suction air quantity can be increased without reducing the ultimate vacuum.
In addition, the present invention provides a negative pressure supply apparatus including an air outlet port connected to a negative pressure source. An air inlet port is open to the atmosphere. A negative pressure port is connected to a negative pressure chamber of a negative pressure device. The apparatus further includes a passage for providing communication between the air outlet port and the negative pressure port. A first check valve allows air to flow through the passage only in the direction from the negative pressure port to the air outlet port. An ejector has an air outlet communicating with the air outlet port, an air inlet communicating with the air inlet port, and a negative pressure outlet communicating with the negative pressure port. A second check valve allows air to flow only in the direction from the negative pressure port to the negative pressure outlet. The negative pressure supply apparatus further includes a control valve for selectively opening or closing either the air outlet or the air inlet of the ejector. The control valve operates in response to the negative pressure at the negative pressure port such that the control valve is open until the negative pressure reaches a predetermined negative pressure, and when the negative pressure has reached the predetermined negative pressure, the control valve is closed rapidly.
With the above-described structure, the control valve is open until the negative pressure at the negative pressure port reaches a predetermined negative pressure. The ejector is operated by the negative pressure from the negative pressure source to supply a negative pressure to the negative pressure port from the negative pressure outlet through the second check valve. When the negative pressure at the negative pressure port has reached the predetermined negative pressure, the control valve is closed to stop the operation of the ejector. Consequently, the negative pressure from the negative pressure source is supplied directly to the negative pressure port through the first check valve. Because the control valve is closed rapidly, the function of the ejector will not be degraded during the period of valve-closing transition by restriction of the flow path by the control valve.
In the negative pressure supply apparatus according to the present invention, the control valve may be disposed on the side of the air inlet with respect to the ejector. With this arrangement, the pressure loss caused by the control valve is minimized, and the efficiency of the ejector is increased.
In the negative pressure supply apparatus according to the present invention, the control valve may be arranged so that the end of its valving member facing the direction in which the control valve moves when it is closed is subjected to a pressure which is lower than that at the other end.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.